In the prior art, it is known to use borates and boric acid as a fire retardant when manufacturing wood products like oriented strand board (hereinafter OSB), particle board (hereinafter PB), and medium density fiberboard (hereinafter MDF).
Flame spreadability or flame spread performance is often used as a measure of the fire retardancy of a given product. There are a number of different kinds of surface flammability tests used to assess the fire retardancy of materials and a description of these is found in a treatise on Analytical Chemistry, Part 3, Volume 4 edited by I. M. Kolthoff, Philip J. Elving, Fred H. Stross, published by John Wiley & Sons, Inc., Copyright 1977, Section D-1: Thermal and Chemical testing, Part iii, Section D-1, Surface flammability measurements for building materials and related products by Herbert W. Eickner, Forest Products Laboratory, US Dept. of Agriculture, Madison, Wis., which is hereby incorporated by reference in its entirety. One of the flame spreading tests is the Steiner tunnel or 25-ft tunnel furnace method (ASTM Standard E84-70), as developed by the Underwriters' Laboratories, as a rating method for measuring surface flammability of building materials. In this test, a flame spread index (FSI) is calculated based on the distance of the flame travel and the rate at which the flame front advances during a specified time of exposure. To have a class A fire rating, which is the best rating, the FSI should be in the range of 0-25. Class B has an FSI range of 30-75, and a Class C fire rating has an FSI range of 80-200. Typically, engineered wood products can only attain a Class C rating whereas pressure treated plywood would have a Class A rating using this test.
Another surface flame spread test is the radiant-panel method, which was developed as a result of seeking a faster laboratory scale testing method than the Steiner tunnel furnace method that would have some predictive correlation with E84 test results. This radiant-panel test method is ASTM Standard E162 (17).
An example of a boric acid dispersion for use as a fire retardant for wood products is found in U.S. Pat. No. 4,801,404 to Dietrich et al. (Dietrich). This patent discloses the use of a granular boric acid in a low-shear mixing process to yield a dispersion of boric acid having an average particle size of about 800 μm. The process uses some alkali base to facilitate the dispersion of the boric acid granules. The preferred molar ratio of boric acid to alkaline agent (namely NaOH) used to form a small amount of borate salt is claimed as being 1.0:0.01 to 1.0:0.20 (or 100:1 to 5:1) and in the one illustrative example, the molar ratio of boric acid/NaOH is 1.0:0.02 (or 50:1).
Dietrich teaches that the use of ground boric acid powders is disadvantageous in producing boric acid dispersions as it indicates that non-uniform particle size distributions are produced and that high viscosities are encountered such that dispersions of lower solids contents are only possible. Dietrich also suggests that the boric acid can be combined with a dispersant but provides no disclosure of any specific dispersant chemistries showing utility. However, the kinds of dispersions being produced by Dietrich are not stable over long time periods; they will hard pack settle and must be kept continuously agitated or be re-agitated at the time of use. Such dispersions would not be suitable for longer term storage in totes or tank trucks for subsequent transport to engineered wood board mills.
U.S. Pat. Nos. 7,354,503, 7,553,538 and 7,651,591 are directed to a fire retardant composite panel product and a method and system for fabricating same and are assigned to Sierra Pine. These patents principally cover an integrated process system located on site at a board mill whereby a boric acid/borax slurry is produced, feed through a high-shear mixing system or a colloid media mill to attrition down the particle size, and the resultant finer particle size material is then fed into a wood fiber line prior to the driers to yield a fire retardant MDF board. Other than a process flow diagram, there is no real process data or processing details provided. No target particle size range is disclosed from their high-shear mixing or media mill process. No dispersion solids range is provided and there is absolutely no mention of any processing additive chemistries.
The stability problem noted above for the dispersions produced in the Dietrich patent also exists when using a boric acid/borax combination like that disclosed in the Sierra Pine patents. Borax is a sodium borate (specifically disodium tetraborate decahydrate but this borate can also exist as lower hydrates such as the pentahydrate). Borate dispersions can crystallize into solidified masses upon standing, especially at higher concentrations and this also leads to a stability problem over time.
Another problem with the use of borates and boric acid is the homogeneity of the fire retardant additive in the wood product. That is, the boric acid or borate particles, as they are dense and large in size, can segregate in the wood product and not be dispersed within the engineered wood product in a homogenous way. This lack of homogeneity can affect the flame spread index (FSI) rating of the wood product such that it cannot achieve a Class A rating in E84 testing. This segregation problem can exist when applying coarse particle size dispersions of borates and boric acid but is particularly problematic when blending in the borates and boric acid in their dry powder or dry granular forms.
Further, even in instances having a fairly good mix of the boric acid or borate particles, in manufacturing processes where a pressing step is used, the fire retardants can subsequently segregate as a result of the pressing step and adequate fire retardancy may only exist on one side of the wood product.
To have maximum flame spread performance, pressure treated plywoods are preferred materials for building use. However, these kinds of plywoods are not without their disadvantages due to cost and due to the additives used during their manufacture that can cause some environmental concerns. Engineered wood products like OSB, particle wood, and MDF do not have the same environmental concerns as pressure treated plywoods as the chemicals used in the plywoods are not used in the engineered wood products. However, current engineered wood products for construction applications are inferior in their fire retardancy as compared to pressure treated plywoods. For example, many engineered wood products would only have a Class C fire rating for flame spread performance.
As such, there is a need to further improve the fire retardancy of engineered wood products so that they can better compete with pressure treated plywoods in the building and construction industry. In addition, efficient and economical methods are needed to meet the fire retardancy needs in other wood related applications such as furniture. The present invention responds to these needs by providing a boric acid dispersion that can improve the flame spread performance properties of engineered wood products and provide new types of engineered wood products as well.